This invention relates to recovery of aqueous sodium base from sodium minerals by contacting sodium minerals such as dry-mined or underground deposits of trona with aqueous acid generated by two- or three-compartment electrodialytic water splitting. More particularly, the invention relates to acid solution mining of subterranean deposits of sodium minerals such as trona or nahcolite to produce an aqueous solution of sodium ions which is subjected to two- or three-compartment electrodialytic water splitting to produce an aqueous hydrogen ion-enriched solution which is recycled to the subterranean deposit, and an aqueous sodium base which may be concentrated for sale or optionally may be used to convert sodium minerals such as trona or nahcolite into soda ash via caustic solution mining and processing or above-ground processing of dry-mined sodium minerals.
Most soda ash (sodium carbonate) produced in the United States is obtained from naturally occurring subterranean trona ore deposits located in southwestern Wyoming. Trona ore consists mainly of sodium sesquicarbonate. Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O, a hydrated sodium carbon-ate, sodium bicarbonate double salt, and normally con-tains 4-13% insoluble impurities. A typical analysis of the crude trona is:
______________________________________ Percent ______________________________________ Na.sub.2 CO.sub.3 41.8 NaHCO.sub.3 33.1 H.sub.2 O 14.1 NaCl 0.04 Na.sub.2 SO.sub.4 0.01 Iron 0.08 Water insolubles 10.87 ______________________________________
The composition of the crude trona corresponds quite closely to that of pure sodium sesquicarbonate except for the impurities present. Shale stringers or beds, present throughout the trona bed, can alter the amount of impurities in different parts of the trona bed.
At the present time, trona deposits are normally mechanically (dry) mined and converted to soda ash by either the sesquicarbonate process or the monohydrate process [the features of which are summarized in U.S. Pat. No. 3,528,766 (Coglaiti et al)]. In the sesquicarbonate process, the trona ore is dissolved in hot aqueous alkali solution and, after separation of the resulting solution from the insolubles, sodium sesquicarbonate is crystallized from solution by cooling. The sesquicarbonate crystals are separated from the mother liquor and then calcined to recover soda ash (anhydrous sodium carbonate). In the monohydrate process, the dry-mined trona ore is first calcined to convert its bicarbonate content to sodium carbonate which is then dissolved in water. After the resulting solution is separated from the insolubles, sodium carbonate monohydrate is precipitated by evaporative crystallization. The monohydrate crystals are separated from the mother liquor and dried to recover soda ash.
Recently, solution mining techniques have been utilized as an alternative to mechanical (dry) mining to recover soda ash from subterranean trona ore deposits. Solution mining of subterranean trona deposits by use of hot water, and various alkaline solutions is well known. For example, U.S. Pat. No. 2,388,009 (Pike) discloses the use of a hot water or hot carbonate solution as the mining fluid. See also U.S. Pat. Nos. 2,625,384 (Pike et al.), 2,847,202 (Pullen); 2,979,315 (Bays); 3,018,095 (Redlinger); 3,050,290 (Caldwell et al.); 3,086,760 (Bays); 3,405,974 (Handley et al) and 4,288,419 (Copenhafer et al.). Solution mining of subterranean trona deposits by the use of aqueous sodium hydroxide is disclosed by U.S. Pat. Nos. 3,184,287 (Gancy), 3,952,073 (Kube) and 4,344,650 (Pinsky et al.). U.S. Pat. No. 4,283,372 (Frint et al.) discloses the use of an aqueous ammonia solution as a mining fluid for trona. These prior art solution mining processes, however, involve substantial energy inputs in manufacturing sodium hydroxide, manufacturing and recycling ammonia, supplying high temperature mining solution, and calcining one or more intermediates.
In addition to trona, nahcolite (predominantly NaHCO.sub.3) and wegscheiderite (predominately Na.sub.2 CO.sub.3.3NaHCO.sub.3) are sodium bicarbonate-containing ores from which it is possible to recover soda ash, after conversion of the bicarbonate to carbonate. Known deposits of nahcolite and wegscheiderite are located primarily in Utah and Colorado. No commercial operations are presently known to be recovering soda ash from these NaHCO.sub.3 -bearing minerals. However, various U.S. patents disclose solution mining of nahcolite. See, for example, U.S. Pat. Nos. 3,779,602 (Beard et al.) and 3,792,902 (Towell et al.), as well as 3,952,073 (Kube) and 4,283,372 (Frint et al.) which disclose basic solution mining of nahcolite and wegscheiderite.
Electrodialysis, as disclosed in U.S. Pat. No. 3,475,122, can be used to produce acid and base from salt and water. Such a process generates H.sup.+ and OH.sup.- ions only at the electrodes and, at the same time, generates H.sub.2 and O.sub.2 (or other electrode oxidation and reduction products). Thus, each equivalent of H.sup.+ and OH.sup.- generated results in an equivalent amount of H.sub.2 and OH.sup.- (or other oxidation and reduction products) being produced.
Electrodialytic water splitting is another method for generating acid and base from salt and water. With a water splitting process, H.sup.+ and OH.sup.- ions can be generated from each of several bipolar membranes arranged between the electrodes without forming H.sub.2 and O.sub.2 at the membrane faces (relatively limited quantities of H.sub.2 and O.sub.2 are formed at the electrodes where oxidation-reduction is taking place). Therefore, electrodialytic water splitting oxidation-reduction products are formed in only small amounts relative to the total amount of hydrogen and hydroxyl ions formed. Consequently, the process of electrodialytic water splitting requires less energy than the process of electrolysis since the energy required to produce H.sub.2 and O.sub.2 from the water and electrolysis must be supplied in addition to the energy needed to produce hydrogen and hydroxyl ions from water.
Electrodialytic water-splitting processes have been employed in the prior art to recover valuable products from dilute soda streams. For example, U.S. Pat. No. 4,082,835 (Chlanda et al.) discloses an electrodialytic process which utilizes two- or three-compartment water-splitters to remove SO.sub.2 from dilute gas streams by means of (a) alkaline solution scrubbing, (b) regeneration of the scrubbing solution and, (c) liberation of concentrated SO.sub.2. This procedure suffers from a number of inherent disadvantages. For example, although one or more of these operations may be effected in a two-compartment water-splitter, the composition of the solutions fed to both compartments is identical. Furthermore, the basic product solutions, e.g., aqueous NaOH, Na.sub.2 SO.sub.3, and Na.sub.2 SO.sub.4 are mixtures and, as such, are normally recycled for further alkaline scrubbing of a SO.sub.2 -containing solution instead of being used in other processes wherein relatively pure products are required. Moreover, the process does not produce soda ash as a product solution. Another electrodialytic process for converting aqueous streams of trona into valuable products is described in U.S. Pat. No. 4,238,305 (Gancy and Jenczewski). In this process, dilute aqueous trona is fed to the acid compartment of an electrodialytic cell for conversion into sodium hydroxide and carbon dioxide. The sodium hydroxide and carbon dioxide products can be recovered and used separately, or combined in another reaction zone to provide soda ash. While relatively effective, this procedure also suffers from certain inherent disadvantages. For example, this process requires that H.sub.2 CO.sub.3 /CO.sub.2 be generated and liberated within the acid compartment thereby increasing the electric power necessary for the electrodialytic process. In addition to the increased energy expenditures, there is also an increase in the cost and complexity of the equipment necessary for effectuating the process.